This invention relates to a digital pulse modulation process, and to a digital pulse modulator and a control system using such a modulator.
Many control devices, such as flow control valves, reaction engines, proportioning mechanism, temperature control switches, three-position switching elements, magnet coils, and the like, which are to be used in control processes and systems, conventionally have only the function states ON/OFF in order to provide an inexpensive and reliable construction or to eliminate undesirable leakage losses. In order to generate an output quantity having a time average which follows a given time-variable input control guide signal, such control devices must be driven by means of pulses having a pulse width, pulse frequency and/or pulse-time range which bears a defined relationship to the input control and, in particular, is directly proportional to it.
Various types of modulators are normally used to convert such a control signal into an equivalent pulse sequence; these include pulse width modulators, pulse frequency modulators, pulse width - pulse frequency modulators (PWPF) or pseudo - rate modulators. For technical applications in closed control loops, not only the type of the pulse sequence and its proportionality to the input signal, but also its phase relationship with the control signal, are critical because the stability and dynamic behavior of a control loop, and thus the quality of the control operation, are substantially dependent on it. It is known that all physical systems which occur in practice ("controlled systems") have an inherent delay phenomenon, (that is, their output response has a negative phase angle rotation relative to their input signal conditions). In those instances where the above-mentioned types of control elements are used to control systems with significant delays, the most advantageous processes for conversion of the control signals are those which generate a positive phase angle rotation (that is, the fundamental component contained in the output pulse pattern has an inherent phase advance relationship with respect to the input signal).
In particular, so-called pseudo-rate modulators which generate an output signal that is proportional not only to the time variable amplitude of the input signal, but also to its rate of change (pseudo rate) have such favorable characteristics. When other types of modulators are used, in order to achieve comparable stability characteristics in a control loop, additional controller networks must be used to generate a phase advance relationship (positive phase angle); such measures, however, would cause a deterioration of the signal-to-noise ratio and would cause additional implementation expenditures.
Recently, digital controllers have been used increasingly for this purpose, so that the use of digital modulators is also considered desirable. The favorable lead-generating characteristics of a pseudo-rate modulator are thus particularly advantageous because additional phase losses are created by the unavoidable use of sample and hold elements; by delay times in the digital computers which may replace the controller networks; as well as by input and output delays, and the like. Such phase losses may have disadvantageous effects on the stability and dynamics of the associated control loops.
Although the simulation of the function elements of an analog pseudo-rate modulator in a digital arithmetic unit is possible in principle, it requires extremely short computing cycles in order to assure a dynamic response that corresponds to that of an analog modulator. A practical comparison of the function characteristics for typical applications shows that cycle rates of from 1 to 2.5 kHz would be necessary to process the modulator routines; that is, sampling rates which are approximately 20 to 50 times higher than for the control loop algorithms would be required, which would result in a correspondingly high load on the process computers and in an undesirable occupancy of the highest priority levels for the computing operations.
U.S. Pat. No. 4,599,697 discloses a satellite attitude control arrangement based on the use of a digital PWPF modulation, which utilizes a process of the initially mentioned type. The satellite itself constitutes the controlled system, with the controlled quantities being attitude angles about the three satellite axes. Measuring devices in the form of earth and sun sensors as well as gyros measure the actual values of the attitude angles and angular velocities. Based on these values, a controller determines deviations from predeterminable desired values and generates error signals in the form of a series of digital sampling values, which values remain constant within each individual sampling period but vary over time--that is, the series of sampling values varies with respect to time.
This system uses a digital PWPF modulation process to convert input error signals into output signals in the form of a sequence of positive and/or negative pulses which have a constant amplitude and a variable pulse width and frequency. This is a simulation process in which each sampling period is divided into a fixed time slot pattern, and the modulator output signals are computed at uniform intervals. In each case, these computations occupy a portion of the sampling period, and the computed pulse sequence is retrieved in the next sampling period and is used to control the control elements, such as attitude control jets, which furnish discrete control pulses in order to maintain the satellite attitude angles at the desired values.
One disadvantage of this known simulation process is that, because of the fixed time slot pattern for each sampling period, the real temporal switching points of the analog modulator whose behavior is to be expressed digitally, are not precisely duplicated. In addition, excessively high computing expenditures are required because many computing cycles are completed between switching points, during which time the simulated output signal does not change. These disadvantages are interrelated in that although the precision in the timing of the switching point may be enhanced by means of a narrower nesting of the time-slot pattern, such a modification also results in an increase of the number of computing cycles per sampling period. Hence, the computing expenditures will rise at the same time, and vice versa.
It is therefore an object of the present invention to provide a digital modulation process which reduces computing expenditures, and at the same time increases the precision in the timing of the switching points. In addition, a digital modulator is to be provided for carrying out a process of this type as well as a control system using such a digital modulator.
These and other objects and advantages are achieved according to the invention by computing the switch-on and switch-off points of the pulses, as well as their operational signs, directly in one operation for each sampling period. According to the invention, the computation (operating cycle) to be carried out at the start of each new sampling period takes place at a very high speed within a time that is very short relative to the duration of the sampling period, so that the complete pulse sequence that is applicable to the current sampling period is available from the beginning. In this manner unnecessary computing expenditures for the computation of conditions between switching points are eliminated and only those points in time are determined at which pulses are to be switched on and off as well as the operational signs which are in each case required for the pulses to be switched on. The computations are carried out in a computing system which comprises arithmetic units, memories and comparators. In addition to the input signals which represent the digitized error signals, modulator parameters which characterize the analog modulator are also supplied to the computing system. The algorithm according to which the processing takes place in the computing system will, of course differ depending on the type of modulator that is to be digitally implemented.
The principal advantage of the process according to the invention is that it permits highly precise computation of the temporal switching points, while at the same time it reduces overall computing expenditures in comparison to the known simulation process. The output signals of the computing system can be stored until the whole pulse sequence that is to be assigned to a sampling period is available. The readout from this memory may then take place with an arbitrary precision with respect to the clock frequency to be used.
In one embodiment of the invention, operation of a pseudo-rate modulator is to be digitally implemented by the process according to the invention. As mentioned above, such a modulator (as well as the respective variant of the process according to the invention) has characteristics which generate a phase advance. Also in the case of this variant, the advantage is naturally obtained that, within one operating cycle, only as many computing cycles are required as the number of switch-on and switch-off points which the corresponding sampling period would have in the case of an analog implementation of the modulator. In the case of a digital implementation of a modulator of the PWPF-type (or of another type), the switch-on and switch-off points as well as their operational signs would have to be computed analogously, specifically on the basis of the mathematical formulae characterizing the respective modulator type. However, by means of modulator versions which differ from the pseudo-rate type, the desired phase lead cannot be achieved in the same manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.